Catalyst for chemical looping combustion

ABSTRACT

A catalyst for use in chemical looping combustion is provided. The catalyst includes a mixture of metal oxides dispersed on a ceramic support. The mixture of metal oxides forms a nickel tungsten oxide (NiWO4) interaction complex which functions as an oxygen carrier in the chemical looping combustion reaction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser.No. 63/017,352, filed on Apr. 29, 2020, the entire disclosure of whichis incorporated herein by reference.

FIELD

The technology described herein relates to industrial processes forgenerating products from basic petroleum products and more specificallyto oxygen carrying catalysts for chemical looping combustion which isused for power generation and synthesis of hydrogen and industrialchemicals.

BACKGROUND

Chemical looping combustion (CLC) is a process used to capture CO₂ andto convert carbonaceous fuel into various products including power,hydrogen, and industrial chemicals. A typical CLC system consists of tworeactors; a fuel reactor and an air reactor. In the fuel reactor, themetal oxide is reduced, and the fuel is completely converted into CO₂and steam. While in the air reactor, the reduced metal oxides areoxidized using air which also produces heat that can be utilized forpower generation. The two separate reactors are used to avoid contact ofthe fuel with other air constituents, which in turns leads to productionof a highly concentrated CO₂ stream.¹

Oxygen carrier catalysts based on individual metal oxides used forchemical looping combustion have various advantages and disadvantages.Hence, the use of oxygen carriers based on mixed metal oxides has beensuggested to combine the advantages of individual metal oxides togetherwith partial mitigation of the disadvantages.²

Investigations of CLC catalysts have been reported over the past twodecades.

In one investigation, WO₃/W was used for methane conversion and thereactivities of the tungsten metals/oxides were found to be enhancedwith the use of the support. Silica, alumina and zirconia supports weretested and it was found that the zirconia-supported catalyst providedexcellent performance in terms of product yield when compared tounsupported WO₃, silica and alumina supported by WO₃.³

In another study, the effect of WO₃ modification of ZrO₂ on Ni-catalyzeddry reforming of biogas was investigated with 8% (w/w) Ni catalyst.Although Ni dispersion and reducibility characteristics were superiorfor the WO₃-modified catalyst relative to non-modified ZrO₂, itscatalytic performance was inferior as a result of enhanced acidity.⁴

A dual catalyst for chemical looping partial oxidation of methane wastested with WO₃-based catalyst modified with Ni over an aluminasupport.^(5,6)

Nickel mixed with WO₃/Al₂O₃ has been designed and proposed to be usedfor HDS (Hydrodesulfurization).⁷

Nickel oxide over zirconia prepared by co-precipitation and furthermodified by tungsten oxide (WO₃) has been investigated in dimerizationof ethylene. The addition of WO₃ to NiO/ZrO₂ resulted in enhancement ofreactivity even at room temperature.⁸

In another investigation, WO₃/ZrO₂ was prepared via incipient wetnessimpregnation and further modified by the addition of Ni with maximum Niloading of 5%. The resultant catalysts were tested for n-butaneisomerization, cyclohexane dehydrogenation and n-octanehydroisomerization-cracking. The Ni addition led to increases in totalacidity and concentration of strong acid sites, providing enhancement ofdehydrogenation and cracking reactions.⁹

There continues to be a need for improved catalysts for use in CLC.

SUMMARY

According to one embodiment, there is provided a catalyst for use inchemical looping combustion. The catalyst comprises a mixture of metaloxides dispersed on a ceramic support, the mixture of metal oxidesforming a nickel tungsten oxide (NiWO₄) interaction complex whichfunctions as an oxygen carrier in the chemical looping combustionreaction.

In some embodiments of the catalyst, the ceramic support is calciumaluminate of formula CaAl₂O₄, silica of formula SiO₂, titanium dioxideof formula TiO₂, perovskite of formula CaTiO₃, alumina of formula Al₂O₃,yttrium dioxide of formula Y₂O₃, barium zirconate of formula BaZrO₃,magnesium aluminate of formula MgAl₂O₄, magnesium silicate of formulaMgSi₂O₄, lanthanum oxide of formula La₂O₃ or zirconia of formula ZrO₂.

In some embodiments of the catalyst, the ceramic support is zirconia offormula ZrO₂.

In some embodiments of the catalyst, the zirconia is calcined at atemperature at or above about 900° C. for at least about 4 hours.

In some embodiments of the catalyst, the mixture of metal oxidesincludes nickel oxide of formula NiO and tungsten oxide of formula WO₃.

In some embodiments, the catalyst comprises between about 25% to about60% NiO (w/w), between about 10% to about 35% WO₃ (w/w) and betweenabout 5% to about 65% ZrO₂ (w/w).

In some embodiments, the catalyst has an oxygen carrying capacity ofabout 4.2% (w/w) to about 15.6% (w/w).

In some embodiments, the catalyst has a Brunauer-Emmett-Teller (BET)surface area between about 4.1 m²/g to about 16.7 m²/g.

In some embodiments, the catalyst has a pore volume of about 0.030 cm³/gto about 0.094 cm³/g.

In some embodiments, the catalyst has an adsorption average pore width(4V/A by BET) between about 225 Å to about 300 Å.

According to another embodiment, there is provided a process forsynthesizing an oxygen carrier catalyst. The process includes the stepsof mixing nickel (II) nitrate hexahydrate (N₂NiO₆.6H₂O) with ammoniummetatungstate and a ceramic support in water and evaporating the water.

In some embodiments of the process, the ceramic support is calciumaluminate of formula CaAl₂O₄, silica of formula SiO₂, titanium dioxideof formula TiO₂, perovskite of formula CaTiO₃, alumina of formula Al₂O₃,yttrium dioxide of formula Y₂O₃, barium zirconate of formula BaZrO₃,magnesium aluminate of formula MgAl₂O₄, magnesium silicate of formulaMgSi₂O₄, lanthanum oxide of formula La₂O₃ or zirconia of formula ZrO₂.

In some embodiments of the process, the ceramic support is zirconia offormula ZrO₂.

In some embodiments of the process, the zirconia is calcined at atemperature at or above about 900° C. for at least about 4 hours.

In some embodiments of the process, the synthesized catalyst includesnickel oxide of formula NiO and tungsten oxide of formula WO₃.

In some embodiments of the process, the synthesized catalyst includesbetween about 25% to about 60% NiO (w/w), between about 10% to about 35%WO₃ (w/w) and between about 5% to about 65% ZrO₂ (w/w).

In some embodiments of the process, the synthesized catalyst has anoxygen carrying capacity of about 4.2% (w/w) to about 15.6% (w/w).

In some embodiments of the process, the synthesized catalyst has aBrunauer-Emmett-Teller (BET) surface area between about 4.1 m²/g toabout 16.7 m²/g.

In some embodiments of the process, the synthesized catalyst has a porevolume of about 0.030 cm³/g to about 0.094 cm³/g.

In some embodiments of the process, the synthesized catalyst has anadsorption average pore width (4V/A by BET) between about 225 Å to about300 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

Various objects, features and advantages of embodiments of the subjecttechnology will be apparent from the following description.

FIG. 1 is a plot of methane CLC performance (3 cycles) for bulk WO₃,calcined zirconia, and WO₃(40%) supported on zirconia generated byco-precipitation and by impregnation.

FIG. 2 is a plot of CLC performance of NiO(20%)-WO₃(20%)/ZrO₂ generatedby co-precipitation and by impregnation.

FIG. 3 is a plot of performance of NiO(20%)/ZrO₂ over 20 cycles ofmethane CLC.

FIG. 4 is a plot of performance of NiO(20%)-WO₃(5%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 5 is a plot of performance of NiO(20%)-WO₃(10%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 6 is a plot of performance of NiO(20%)-WO₃(15%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 7 is a plot of performance of NiO(20%)-WO₃(20%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 8 is a plot of performance of NiO(20%)-WO₃(25%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 9 is a plot of performance of NiO(20%)-WO₃(30%)/ZrO₂ (impregnation)over 20 cycles of methane CLC.

FIG. 10 is a plot of performance of NiO(20%)-WO₃(35%)/ZrO₂(impregnation) over 20 cycles of methane CLC.

FIG. 11 is a plot of performance of impregnated single metal oxideoxygen carrier catalyst and impregnated dual metal oxide oxygen carriercatalyst for 20 cycles of methane CLC.

FIG. 12 is a plot of performance of NiO(60%)-WO₃(25%)/ZrO₂(impregnation) for 20 cycles of methane CLC.

FIG. 13 is a plot of performance of NiO(65%)-WO₃(25%)/ZrO₂(impregnation) for 20 cycles of methane CLC.

FIG. 14 is a plot of performance of NiO(70%)-WO₃(25%)/ZrO₂(impregnation) for 20 cycles of methane CLC.

FIG. 15 is a plot of performance of unsupported NiO(75%)-WO₃(25%)(impregnation) for 20 cycles of methane CLC.

FIG. 16 shows a series of X-ray diffraction (XRD) analyses of calcinedzirconia, bulk WO₃, WO₃ (40%)/ZrO₂ (impregnation), and WO₃(40%)ZrO₂(co-precipitation).

FIG. 17 shows a pair of XRD analyses of NiO(20%)-WO₃(20%)/ZrO₂(impregnation), and NiO(20%)-WO₃(20%)/ZrO₂ (co-precipitation).

FIG. 18 shows a series of XRD analyses of supported dual oxygen carriercatalysts generated by impregnation with 20% NiO and variablepercentages of WO₃.

FIG. 19 shows a series of XRD analyses of supported dual oxygen carriercatalysts generated by impregnation with 25% WO₃ and variablepercentages of NiO.

DETAILED DESCRIPTION Rationale

Carbon (coke) formation associated with Ni-based oxygen carriers resultsin transportation of carbon to the air reactor with the oxygen carriers.Then the carbon is converted to CO₂ in the air reactor, resulting indegradation of CO₂ separation efficacy. If methane is used as a fuel inCLC, carbon can be formed through methane decomposition (equation 1) orthe Boudouard reaction (equation 2).¹⁰

CH₄→C+2H₂  (eq 1)

2CO→+C+CO₂  (eq 2)

The inventors recognized that further investigation of WO₃-modifiedsupports such as ZrO₂ further modified with oxides of nickel would be aworthwhile endeavor which could lead to improvements in catalyticreactivity with enhanced oxygen carrying capacity and mitigation offormation of carbon (coke) on the surface of the catalyst.

INTRODUCTION

Experiments leading to identification of useful catalyst compositionsfor CLC will now be described hereinbelow with reference to data shownin tables and figures. A number of possible alternative features areintroduced during the course of this description. It is to be understoodthat, according to the knowledge and judgment of persons skilled in theart, such alternative features may be substituted in variouscombinations to arrive at different embodiments.

A novel nickel-based oxygen carrier catalyst for chemical loopingcombustion (CLC) is described. Zirconia (ZrO₂) was selected as thesupport due to its stable performance and high melting point (2715° C.).However, it is to be understood that other ceramic supports may be usedinstead of zirconia. Such alternative ceramic supports include but arenot limited to calcium aluminate of formula CaAl₂O₄, silica of formulaSiO₂, titanium dioxide of formula TiO₂, perovskite of formula CaTiO₃,alumina of formula Al₂O₃, yttrium dioxide of formula Y₂O₃, bariumzirconate of formula BaZrO₃, magnesium aluminate of formula MgAl₂O₄,magnesium silicate of formula MgSi₂O₄ and lanthanum oxide of formulaLa₂O₃. Tungsten oxide is used as a promoter to facilitate the formationof a stable NiWO₄ interaction complex. The synthesized oxygen carriercatalyst is intended to be used in CLC to enhance the reactivity andmitigate the formation of carbon on the catalyst surface. The catalystsdescribed herein have excellent performance relative to the individualmetal oxides. Methods of synthesizing the catalyst and evaluating thecatalyst for the CLC process are also described.

Materials and Methods

Metal Precursors—Tungstic acid (H₂WO₄ powder) was used as a source ofbulk WO₃. Nickel (II) nitrate hexahydrate (N₂NiO₆.6H₂O) was used as asource of nickel for the impregnation and co-precipitation methods usedto prepare the oxygen carrier catalysts. Ammonium metatungstate((NH₄)₆(H₂W₁₂O₄₀)) was used as a source of tungsten for the impregnationmethod. Tungsten (VI) chloride (WCl₆) was used as a source of tungstenfor the co-precipitation method. For the support, zirconium (IV)oxynitrate hydrate (N₂O₇Zr.xH₂O) was used as a support precursor for theco-precipitation method, and commercial zirconia (zirconium (IV) oxide(ZrO₂)), was used for the impregnation method. All chemicals werepurchased from Sigma Aldrich, Canada, except for the commercial zirconiasupport, which was purchased from Alfa Aesar. All compounds were usedwithout modifications unless otherwise specified hereinbelow.

Bulk tungsten trioxide (WO₃) was prepared by thermal decomposition oftungstic acid (H₂WO₄) in powder form at 800° C. for 4 hours aspreviously described.^(5,6)

The WO₃/ZrO₂ used in this investigation was prepared as previouslydescribed.^(3,11-13)

In one preparation method, the WO₃—NiO over zirconia catalyst wasprepared by co-precipitation as previously described^(5,6) withmodification of the calcination temperature to 900° C. to provideenhanced stability of the catalyst.

In another preparation method, the WO₃—NiO over zirconia catalyst wasprepared using the impregnation method as previously described¹¹ withincorporation of the nickel precursor initially derived from theNi(NO₃)₃.6H₂O, with modification of the calcination temperature to 900°C.

Bulk WO₃ was prepared by thermal decomposition and the mono and dualcatalysts were prepared using co-precipitation and impregnation methods.Dual and mono catalysts were prepared using the co-precipitation methodand the specific catalyst composition was determined.

For the preparation of WO₃/ZrO₂ using the co-precipitation method,ethanol was used to dissolve tungsten (VI) chloride (WCl₆) and zirconium(IV) oxynitrate hydrate (N₂O₇Zr.xH₂O) (since water cannot be used forthe dissolution of WCl₆). Sodium hydroxide (NaOH) was then added to themixture to raise the pH to range between 8-9, as verified using a pHmeter. The addition of the base (NaOH) resulted in the formation ofprecipitates of the supported oxygen carriers. After 2 hours ofcontinuous mixing, the solution was filtered using filter paper, andwashed with distilled water to remove sodium and chloride compounds fromthe sample. Then the sample was dried at 80° C. for 1 day, followed bysample grinding and calcination at 900° C. for 4 hours.

For NiO—WO₃/ZrO₂ co-precipitation, the nickel precursor nickel (II)nitrate hexahydrate (N₂NiO₆.6H₂O) was added to ethanol with the otherprecursors according to the required amount of each composition(composition is based on weight percentages for the total oxygen carrieri.e. metal+support), and the same procedure was followed for theremaining preparation steps.

Preparation of dual and mono catalysts using the impregnation method wasconducted by first determining the specific catalyst composition.

For the preparation of WO₃/ZrO₂ using the impregnation method, distilledwater was used to soak ammonium metatungstate ((NH₄)₆(H₂W₁₂O₄₀)) andcommercial zirconia (ZrO₂). The solution was continuously mixed for 24hours and then the sample was heated to evaporate the water bymaintaining the mixture at a temperature of 75° C. The sample was thenplaced in an oven at 120° C. for 1 day, to promote complete drying,followed by sample grinding and calcination at 900° C. for 4 hours.

For NiO—WO₃/ZrO₂-impregnation, the nickel precursor nickel (II) nitratehexahydrate (N₂NiO₆.6H₂O) was added to water with (NH₄)₆(H₂W₁₂O₄₀) andZrO₂, to provide the required amounts for each composition. The solutionwas continuously mixed for 24 hours and then the sample was heated toevaporate the water by maintaining the mixture at a temperature of 75°C. The sample was then placed in an oven at 120° C. for 1 day, topromote complete drying, followed by sample grinding. The pure zirconiaused to prepare the catalysts of various compositions was calcined at900° C. for 4 hours.

All catalyst compositions described herein are based on weightpercentages for the complete oxygen carrier catalyst. For example, it isto be understood that a composition indicated as “NiO(20%)-WO₃(20%)ZrO₂”will include 20% (w/w) NiO, 20% (w/w) WO₃ and 60% (w/w) ZrO₂.

Characterization of Catalysts and CLC Testing—The oxygen carriercatalysts were tested to assess their reduction and oxidation cycles forthe CLC process using a PerkinElmer 8000 TGA (thermogravimetricanalyzer) by loading 10 mg of oxidized catalyst into a platinum crucibleand placing it in the designated sample holder at 50° C. Then, theanalyzer was programmed to raise the temperature inside the chamber from50° C. to 850° C. at a heating rate of 100° C./min with a nitrogen flowrate of 20 mL/min. The temperature was kept at 850° C. for 3 minutesbefore starting the CLC cycles. To reduce the oxygen carrier catalyst,the entering stream was switched to fuel. The fuel flow was controlledby a gas mixing device (GMD 8000 PerkinElmer) which was set to introduce40% (vol.) of the entering flow of methane and 60% nitrogen (for safetyreasons) with a total flow rate of 20 mL/min. The fuel oxidation(catalyst reduction) continued for 5 minutes before purging the chamberwith nitrogen for 5 minutes at a flow rate of 20 mL/min. After that, thecatalyst oxidation cycle was performed by switching the inlet stream toair at 20 mL/min. These three steps form one cycle (catalyst reduction,nitrogen purging, and catalyst oxidation), thereby completing thereduction and oxidation cycle. The reduction and oxidation cycle wasrepeated as required before cooling off the thermogravimetric analyzerto 50° C. once again. The sample weight percentage change was recordedwith the increase in the weight of the catalyst during oxidation beingdue to oxygen sorption by the sample and weight loss during samplereduction being due to loss of oxygen for fuel combustion.

Characterization of the oxygen carriers by X-ray powder diffraction wasperformed using a Rigaku X-ray diffractometer (Multiflux 2 kW Coppertarget). The samples were placed horizontally on the sample holder. Thesamples were irradiated by Cu k-alpha radiation (wavelength of 1.5406 Å)to excite characteristic x-rays with two-theta diffraction anglesranging from 20-80 degrees, a 0.02 degree step, and a scanning rate of 2degrees per minute. The equipment was operated with 40 kV voltage and 40mA current. XRD patterns of various samples were obtained and analyzedfor phase identification using the Jade 6 XRD MDI library.

Surface area and porosimetry measurements were performed using an ASAP2020 porosimeter. Firstly, samples were degassed at 120° C. for 120minutes to prepare the sample for analysis be removing any adsorbedsurface moisture. Then, sample analysis was performed with N₂adsorption-desorption in the analysis chamber, where the sample wassubmerged in liquid nitrogen.

The experimental oxygen carrying capacity was calculated as:

$\begin{matrix}{R_{\exp} = {\frac{W_{ox} - W_{red}}{W_{ox}} \times 100\%}} & \left( {{eq}\mspace{14mu} 3} \right)\end{matrix}$

Where: W_(ox) is the weight of fully oxidized oxygen carrier catalyst,W_(red) is the weight of reduced oxygen carrier catalyst, andR_(exp) is the experimental oxygen carrying capacity.

TABLE 1 Oxygen Carrying Capacity for Unsupported and Supported Tungstenoxide and Dual Oxygen Carrier Catalyst with Varying WO₃ Loading OxygenCarrying Oxygen Carrier Capacity (%) Bulk WO₃ 0.175 Calcined Zirconia0.023 WO₃(40%)/ZrO₂- 0.414 Co-Precipitation WO₃(40%)/ZrO₂- 0.085Impregnation NiO(20%)—WO₃(20%)/ZrO₂- 6.639 Co-PrecipitationNiO(20%)—WO₃(20%)/ZrO₂- 5.825 Impregnation NiO(20%)/ZrO₂- 3.389Impregnation NiO(20%)—WO₃(5%)/ZrO₂- 3.188 ImpregnationNiO(20%)—WO₃(10%)/ZrO₂- 4.200 Impregnation NiO(20%)—WO₃(15%)/ZrO₂- 4.187Impregnation NiO(20%)—WO₃(20%)/ZrO₂- 5.825 ImpregnationNiO(20%)—WO₃(25%)/ZrO₂- 6.484 Impregnation NiO(20%)—WO₃(30%)/ZrO₂- 5.940Impregnation NiO(20%)—WO₃(35%)/ZrO₂- 5.847 Impregnation

The first cycle data obtained for oxygen carrying capacity ofunsupported tungsten oxide, supported tungsten oxide viaco-precipitation and impregnation along with dual metal oxide oxygencarrier catalyst with different percentages of WO₃ loading are listed inTable 1. The oxygen carrying capacity of bulk WO₃ is 0.175%, and WO₃supported by co-precipitation has a capacity of 0.414% which higher thanthe capacity of the equivalent catalyst prepared by impregnation(0.085%). The zirconia was tested for its contribution towards thereaction, and it showed very low reactivity.

The co-precipitated dual oxygen carrier catalyst NiO(20%)-WO₃(20%)/ZrO₂has higher oxygen carrying capacity (6.639%) than the impregnatedcatalyst (5.825%). However, other factors such as stability and cokeformation favor the impregnation method of preparation as discussedhereinbelow with respect to stability testing.

The impregnation method was used to further investigate the optimumcompositions of the dual oxygen carrier catalyst. Optimal WO₃ loadingwas studied by starting with the single NiO/ZrO₂ oxygen carrier catalystand then increasing the percentage of WO₃ loading. The single oxygencarrier catalyst NiO(20%)/ZrO₂ showed oxygen carrying capacity of 3.389%with coke formation. Adding 5% of WO₃ to the oxygen carrier resulted in3.188% oxygen carrying capacity for the NiO(20%)-WO₃(5%)/ZrO₂. However,further increasing the WO₃ loading was found to improve the oxygencarrying capacity to 4.200% and 4.187% for NiO(20%)-WO₃(10%)/ZrO₂ andNiO(20%)-WO₃(15%)/ZrO₂, respectively. Coke formation was found topersist up to WO₃ loading of 15%. Increasing the WO₃ loading to 20% inNiO(20%)-WO₃(20%)/ZrO₂ resulted in elimination of coke formation andimproved the oxygen carrying capacity to 5.825%. Further increases ofthe WO₃ percentage provided coke-free oxygen carrier catalysts andimproved CLC performance. The optimum WO₃ loading was found to be 25%resulting in 6.484% capacity. Increasing the level of WO₃ loading beyondthis point decreases the oxygen capacity as shown in Table 1.

The NiO loading was varied for the optimal 25% WO₃ loading toinvestigate the effect of the NiO loading on the oxygen carryingcapacity. The results are shown in Table 2.

TABLE 2 Oxygen Carrying Capacity of Carriers with Varying NiO Loadingand WO₃ Loading of 25% over ZrO₂ Oxygen Carrying Oxygen Carrier Capacity(%) NiO(5%)—WO₃(25%)/ZrO₂- 3.685 Impregnation NiO(10%)—WO₃(25%)/ZrO₂-4.232 Impregnation NiO(15%)—WO₃(25%)/ZrO₂- 4.207 ImpregnationNiO(20%)—WO₃(25%)/ZrO₂- 6.484 Impregnation NiO(25%)—WO₃(25%)/ZrO₂- 7.017Impregnation NiO(30%)—WO₃(25%)/ZrO₂- 8.068 ImpregnationNiO(35%)—WO₃(25%)/ZrO₂- 8.963 Impregnation NiO(40%)—WO₃(25%)/ZrO₂-11.047 Impregnation NiO(45%)—WO₃(25%)/ZrO₂- 12.158 ImpregnationNiO(50%)—WO₃(25%)/ZrO₂- 13.383 Impregnation NiO(55%)—WO₃(25%)/ZrO₂-14.420 Impregnation NiO(60%)—WO₃(25%)/ZrO₂- 15.642 ImpregnationNiO(65%)—WO₃(25%)/ZrO₂- 17.594 Impregnation NiO(70%)—WO₃(25%)/ZrO₂-17.970 Impregnation NiO(75%)—WO₃(25%)- 19.490 Impregnation

Generally, increasing the NiO loading in the oxygen carrier enhances theoxygen carrier capacity. However, coke formation was observed for theNiO(65%)-WO₃(25%)/ZrO₂, (70%)-WO₃(25%)/ZrO₂ and (75%)-WO₃(25%) samplesduring CLC testing. This will be discussed further hereinbelow.

Stability of the Oxygen Carrier Catalysts in CLC—FIG. 1 shows themethane CLC performance over 3 cycles for calcined zirconia, bulk WO₃,and WO₃ supported on zirconia by impregnation and co-precipitation.Introduction of nitrogen produces the horizontal lines at the beginningof the experiment and between oxidation and reduction. The weight lossthat occurs directly after the end of nitrogen flow is due to theintroduction of methane, which promotes reduction of the oxygen carriercatalyst and weight loss. In cases where carbon formation occurs, theweight increase is seen in the second smaller peak. In the oxidationstep, air is introduced into the system to combust the carbon that waspreviously formed during reduction of the catalyst, resulting in adecrease in the weight of the catalyst as a result of release of carbon.Then, formation of the metal oxide in the catalyst appears as weightgain. The number of cycles was repeated as many times as needed to testthe oxygen carrier catalyst CLC performance and carbon deposition on thesamples.

At first, the WO₃ bulk sample (prepared from thermal decomposition oftungstic acid) was tested for CLC in a TGA analysis over 3 cycles (FIG.1). It was found that this sample has very low oxygen carrying capacityof less than 0.20%. To form a baseline for the supported samples,calcined commercial zirconia was analyzed by TGA as shown in FIG. 3, andit was found that, for each cycle, the oxygen carrying capacity is quotelow due to the difficult reduction of ZrO₂ that requires very hightemperatures.

The performance of catalysts formed of WO₃ dispersed on ZrO₂ prepared bythe co-precipitation method and by the impregnation method are shown inFIG. 3. The loading of WO₃ in the total oxidized form of the catalystused is 40% (w/w). The R_(exp) value for the co-precipitated OC wasabout 0.78% which is higher than that of the impregnated catalyst.However, for both the co-precipitated and the impregnated WO₃ samplestogether with the bulk WO₃ showed very poor methane CLC performancebecause WO₃ has low reactivity in a methane environment.⁶

After confirming the low reactivity of WO₃ for the methane CLCexperiment, samples of NiO—WO₃ dispersed on ZrO₂ were tested as shown inFIG. 2. Oxygen carrier catalysts with a composition of 20% NiO, 20% WO₃and 60% ZrO₂ were prepared by co-precipitation and impregnation andcompared. In FIG. 2, it can be seen that the co-precipitated dualmetallic oxide oxygen carrier catalyst has an oxygen carrying capacityof 6.77% for the first cycle, but a large amount of carbon was formed onthis sample (about 20.37% (w/w)). With the start of air oxidation, theoxygen consumed to burn off the deposited carbon leads to a decrease inthe weight of the catalyst, followed by formation of metal oxide thatleads to a further increase in weight. For the second cycle, the oxygencarrying capacity was slightly reduced to 6.57% and carbon deposition bymethane decomposition was also observed (about 6.43% of the oxidizedsample). Similar to the oxidation of the first cycle, carbon was burntin the second oxidation resulting in weight reduction of the sample thatfollowed by formation of metal oxide and a weight increase. For thethird cycle, the oxygen carrying capacity was observed to be 6.73% andthe carbon formed for this cycle was 4.17% of the oxidized sampleweight. The oxidation of this cycle led to the release of carbon andconsumption of oxygen to form a metal oxide. A similar trend wasobserved for the other cycles and the oxygen carrying capacity for the20th cycle slightly decreased to 6.44% and the carbon amount was 2.10%.

Also indicated in FIG. 2, the impregnated dual metallic oxide oxygencarrier catalyst showed oxygen carrying capacity of 6.00% for the firstcycle of methane CLC, and this performance was found to be more stablecompared to the co-precipitated dual catalyst with no formation ofcarbon observed. However, a decrease in the oxygen carrying capacity to5.63% was observed in the second cycle, and 5.68% in the third cycle.The performance was stable for this catalyst, and a similar trend wasobserved for the remaining cycles. In the 20^(th) cycle, the oxygencarrying capacity was 5.81%. The catalyst formed by impregnation showedsuperior methane CLC performance compared to the co-precipitated samplein terms of stability and less carbon formation. Therefore, theimpregnation method was selected over the co-precipitation method forpreparing the oxygen carrier catalyst used in subsequent investigationsdescribed hereinbelow. Previous reports have indicated that theimpregnation method is effective because it permits fast deposition ofmetals with high loadings and controllable preparation steps. Moreover,the preparation steps used for lab-scale investigations can beconveniently scaled up to a commercial scale. However, one principaldrawback of the impregnation method is that loaded metal isnon-uniformly distributed through the support.¹⁴

Different WO₃ loadings were added to the NiO/ZrO₂ catalyst using theimpregnation method to study the effect of the dual oxygen carriercatalyst on CLC performance. The NiO/ZrO₂ catalyst with 20% (w/w)loading of NiO was used as a baseline catalyst modified by addition ofWO₃ in the following percentages: 5%, 10%, 15% and 20%. The performanceof NiO(20%)ZrO₂ for 20 CLC cycles is shown in FIG. 3, while theperformance of a series of dual metallic oxygen carrier catalysts (bychanging WO₃ loading) for 20 cycles of methane CLC are shown in FIGS.4-10.

The TGA profile of the base material NiO(20%)/ZrO₂ (FIG. 3) showedoxygen carrying capacity of about 3.39% in the first cycle. Thiscapacity remained relatively constant over the 20 cycles. However, forthe NiO(20%)/ZrO₂ sample, formation of carbon was observed on the sampleduring the reduction of the catalyst as noted above. The percentage ofthe carbon formed in the first cycle was 1.64% (w/w) based on theoxidized sample and 1.70% (w/w) based on the reduced sample. Theformation of carbon decreased with time and it was found to be 0.64% inthe 20^(th) cycle. In the proposed CLC process, the carbon formed on theoxygen carrier catalyst in the fuel reactor is transported with thecatalyst to the air reactor, where the carbon is released in the form ofCO₂. This causes a reduction in CO₂ capture efficiency.¹⁰

The addition of 5% WO₃ forms the NiO(20%)-WO₃(5%)ZrO₂ dual oxygencarrier catalyst where the performance for 20 cycles is shown in FIG. 4.The oxygen capacity for this catalyst was found to be 3.20% in the firstcycle. The oxygen carrying capacity oscillated around this value overthe remaining cycles. However, carbon (coke) is formed on the catalyst,in an amount of 0.26% for the first cycle. The amount of carbon formedincreased up to 0.49% for the second cycle (weight percentage based onthe oxidized oxygen carrier catalyst). The amount of coke oscillatedbetween 0.50% and 0.60%, indicating a reduction in the amount of carbonformation compared to the single metallic oxygen carrier catalyst.

Increasing the loading of WO₃ to 10% forms the NiO(20%)-WO₃(10%)/ZrO₂dual oxygen carrier catalyst. The performance of this catalyst formethane CLC over 20 cycles is shown in FIG. 5. The oxygen carryingcapacity for the first cycle was 4.2%, showing 20% greater oxygencapacity than the single metallic oxide (NiO(20%)/ZrO₂), The carbonformation for the first cycle was 0.60%. However, this percentageincreased to 0.81% in the second cycle, and significantly decreased overthe remaining cycles showing only 0.20% carbon formation in the 20^(th)cycle. On the other hand, the oxygen capacity increased in the secondcycle reaching 4.24%, and later increased to 4.50% and remainedrelatively constant until the 20^(th) cycle.

The performance of NiO(20%)-WO₃(15%)/ZrO₂ is shown in FIG. 6. The oxygencarrying capacity in the first cycle was 4.14% with 0.56% carbon formedon the catalyst. This carbon was burnt in the first oxidation step, andlow carbon formation was observed for the remaining cycles (0.27% at the20^(th) cycle). However, the oxygen capacity increased to 4.42%, in thesecond cycle and remained relatively constant until 20^(th) cycle(4.40%).

The performance of NiO(20%)-WO₃(20%)/ZrO₂ is shown in FIG. 7.Importantly formation of carbon was not observed. The oxygen carryingcapacity in the first cycle was 6.00%. However, this capacity dropped to5.63% in the second cycle and then increased in the remaining cycles.The oxygen carrying capacity was 5.81% in the 20th cycle.

Similar performance was observed for NiO(20%)-WO₃(25%)/ZrO₂,NiO(20%)-WO₃(30%)/ZrO₂ and NiO(20%)-WO₃(35%)/ZrO₂ samples with theoxygen carrying capacity beginning to decrease when the loading of WO₃exceeded 25%, as observed in FIGS. 8-10.

A comparison between the baseline oxygen carrier catalyst(NiO(20%)/ZrO₂) and the dual oxygen carrier catalyst(NiO(20%)-WO₃(25%)/ZrO₂) is shown in FIG. 11. Importantly, carbonformation was not observed for the dual oxygen carrier catalyst(NiO(20%)-WO₃(25%)/ZrO₂). Moreover, the reactivity of the dual oxygencarrier catalyst (NiO(20%)-WO₃(25%)/ZrO₂) is higher than that of thesingle oxygen carrier catalyst (NiO(20%)/ZrO₂). In the first cycle, theoxygen carrying capacity of the dual oxygen carrier was found to be 75%higher than that of the single oxygen carrier catalyst. However, thereactivity of the single oxygen carrier is stable throughout the 20cycles, while the reactivity of the dual oxygen carrier catalystdecreases slightly for the first 3 cycles and then remains stable forthe remaining cycles. The oxygen carrying capacity of the dual oxygencarrier catalyst is much higher than that of the single oxygen carriercatalyst for each cycle.

Further testing was conducted to assess the effect of changing theamount NiO loading while keeping the amount of WO₃ loading constant at25%, which represents an optimal amount of loaded WO₃. The oxygencarrier catalysts range from 5% NiO to 75% NiO loading. TheNiO(5%)-WO₃(25%)/ZrO₂ sample through NiO(60%)-WO₃(25%)/ZrO₂ sampleshowed no carbon formation for 20 cycles of CLC (data not shown). Theaddition of 65% loading of NiO in the NiO(65%)-WO₃(25%)/ZrO₂ and 75% NiOloading in the NiO(70%)-WO₃(25%)/ZrO₂, as well as the unsupportedNiO(75%)-WO₃(25%) sample showed formation of carbon during the 20 cyclesof CLC testing. Data indicating CLC performance forNiO(60%)-WO₃(25%)/ZrO₂, NiO(65%)-WO₃(25%)/ZrO₂, NiO(70%)-WO₃(25%)/ZrO₂and unsupported NiO(75%)-WO₃(25%) are shown in FIGS. 14, 15, 16 and 17,respectively.

Porosimetry and Surface Area Analyses—Porosimetry and surface areaanalyses for the single oxide oxygen carrier catalysts and dual oxygencarrier catalysts with varying amounts of WO₃ loading are shown in Table3.

TABLE 3 Porosimetry and Surface Area Analysis for Unsupported andSupported Tungsten Oxide and Dual Oxygen Carrier Catalyst with VaryingLoading of WO₃ Adsorption Average BET Pore Pore Width Surface Volume(4V/A by Oxygen Carrier Area (m²/g) (cm³/g) BET) Å Bulk WO₃ 1.43790.0025 71 Calcined Zirconia 2.9358 0.0129 175 WO₃(40%)/ZrO₂- 2.31910.0119 206 Co-Precipitation WO₃(40%)/ZrO₂- 2.1415 0.0078 146Impregnation NiO(20%)—WO₃(20%)/ZrO₂- 8.0140 0.0466 233 Co-PrecipitationNiO(20%)—WO₃(20%)/ZrO₂- 6.4671 0.0486 300 Impregnation NiO(20%)/ZrO₂-6.6557 0.0403 242 Impregnation NiO(20%)—WO₃(5%)/ZrO₂- 6.2038 0.0459 296Impregnation NiO(20%)—WO₃(10%)/ZrO₂- 4.1433 0.0300 289 ImpregnationNiO(20%)—WO₃(15%)/ZrO₂- 5.8857 0.0419 285 ImpregnationNiO(20%)—WO₃(20%)/ZrO₂- 6.4671 0.0486 300 ImpregnationNiO(20%)—WO₃(25%)/ZrO₂- 5.0294 0.0351 279 ImpregnationNiO(20%)—WO₃(30%)/ZrO₂- 6.8198 0.0380 223 ImpregnationNiO(20%)—WO₃(35%)/ZrO₂- 6.1018 0.0361 237 Impregnation

The Brunauer-Emmett-Teller (BET) surface area calculated for unsupportedand supported tungsten oxide and dual oxygen carrier catalysts withdifferent tungsten loading is shown in Table 3. Bulk WO₃ was found tohave the smallest surface area. The calcined commercial zirconia supporthas a surface area of 2.94 m²/g. This surface area was found to decreasewhen the WO₃ was dispersed on the support using both co-precipitationand impregnation methods. However, WO₃(40%)/ZrO₂ prepared using theimpregnation method was found to have a smaller surface area than thecatalyst prepared using the co-precipitation method. When comparing thedispersion of NiO on zirconia with the dispersion of WO₃, it can be seenthat the NiO(20%)/ZrO₂ prepared by impregnation has a greater surfacearea than the zirconia support. For the dual metallic oxygen carriercatalyst (NiO(20%)-WO₃(20%)/ZrO₂), the sample prepared byco-precipitation has a higher BET surface area than the same catalystprepared by impregnation. For comparison of the different WO₃ loadingsin the NiO(20%)/ZrO₂ sample, the catalyst with 5% WO₃ slightly decreasesthe BET surface area from 6.66 m²/g to 6.20 m²/g. Further increasingloading of WO₃ to 10% significantly decreases the surface area to 4.14m²/g. Then, for the 15% and 20% WO₃ loadings, the surface area increasesto 5.89 m²/g and 6.47 m²/g, respectively. The NiO(20%)-WO₃(25%)/ZrO₂surface area was found to be lower than that of NiO(20%)-WO₃(20%)ZrO₂.Further increases in WO₃ loading to 30% increases the surface area to6.8198 m²/g, but then additional loading of WO₃ causes a decrease insurface area to 6.1018 m²/g.

The pore volume and the adsorption average pore width of the testedsamples are also shown in Table 3. Generally, a trend similar to thesurface area trend was observed. Bulk WO₃ has the lowest pore volume andpore width among all measurements. The calcined commercial zirconia hasa pore volume of 0.0129 cm³/g and an average pore width of 175 Å.Samples of WO₃ supported on commercial zirconia have a smaller porevolume regardless of the preparation method. However, the average porewidth is higher for the sample prepared by co-precipitation (206 Å). Thedual metallic oxygen carrier catalyst (NiO(20%)-WO₃(20%)ZrO₂) preparedby impregnation has a greater pore width (300 Å) compared to the samesample prepared by co-precipitation (233 Å). For the dual metallic OCprepared by impregnation, the NiO(20%)-WO₃(10%) catalyst was found tohave a small pore volume of 0.03000 cm³/g, and increasing the amount ofloading of WO₃ did not significantly change the pore volume. However,these pore volumes remain generally higher than the pore volume ofNiO(20%)/ZrO₂ (0.0403 cm³/g).

Porosimetry and surface area analyses for carriers with varying NiOloadings and WO₃ loading of 25% supported by ZrO₂ are shown in Table 4.

TABLE 4 Porosimetry and Surface Area Analysis for Supported TungstenOxide and Dual Oxygen Carrier Catalyst with Varying Loading of NiO and25% WO₃ Adsorption Average BET Pore Pore Width Surface Volume (4V/A byOxygen Carrier Area (m²/g) (cm³/g) BET) Å NiO(5%)—WO₃(25%)/ZrO₂- 3.04930.0109 144 Impregnation NiO(10%)—WO₃(25%)/ZrO₂- 3.7899 0.0188 198Impregnation NiO(15%)—WO₃(25%)/ZrO₂- 6.0964 0.0397 261 ImpregnationNiO(20%)—WO₃(25%)/ZrO₂- 5.0294 0.0351 279 ImpregnationNiO(25%)—WO₃(25%)/ZrO₂- 7.9521 0.0530 267 ImpregnationNiO(30%)—WO₃(25%)/ZrO₂- 8.8176 0.0607 276 ImpregnationNiO(35%)—WO₃(25%)/ZrO₂- 10.3766 0.0692 267 ImpregnationNiO(40%)—WO₃(25%)/ZrO₂- 10.9212 0.0686 251 ImpregnationNiO(45%)—WO₃(25%)/ZrO₂- 11.614 0.0631 217 ImpregnationNiO(50%)—WO₃(25%)/ZrO₂- 12.6393 0.0735 233 ImpregnationNiO(55%)—WO₃(25%)/ZrO₂- 13.8885 0.0916 264 ImpregnationNiO(60%)—WO₃(25%)/ZrO₂- 16.7199 0.0939 225 ImpregnationNiO(65%)—WO₃(25%)/ZrO₂- 16.8747 0.0908 215 ImpregnationNiO(70%)—WO₃(25%)/ZrO₂- 15.6013 0.0847 217 ImpregnationNiO(75%)—WO₃(25%)- 14.1919 0.0746 210 Impregnation

The surface area of dual oxygen carrier catalysts with varying amountsof NiO loading was found to increase with increasing the NiO loadingfrom 5% up to 65% with the highest surface area of 16.8747 m²/g for theNiO(65%)-WO₃(25%)ZrO₂. Moreover, the surface area is 15.6031 m²/g forNiO(65%)-WO₃(25%)/ZrO₂ with only 5% of the sample comprising the ZrO₂support. A further decrease of the surface area to 14.1919 m²/g wasobserved for the unsupported NiO(75%)-WO₃(25%).

Regarding the pore volume of the oxygen carriers with varying NiOloading and WO₃ loading of 25%, the general trend observed was anincrease of pore volume with NiO loading peaking at 60% NiO loading(0.0939 cm³/g) after which the pore volume decreases with increasing NiOloading.

The average pore size of the oxygen carrier catalysts with varying NiOloading and WO₃ loading of 25% was found to increase with increasing theamount of NiO loading between 5% and 20%. However, further increases inNiO loading resulted in fluctuations in the average pore size with adecreasing trend with increasing in NiO loading. The observed change ofthe BET surface area and pore structure of the samples could be relatedto effects arising from the preparation methods and the change of thephases formed on oxygen carrier catalysts, as outlined demonstratedhereinbelow with respect to the results of XRD analyses).

X-Ray Powder Diffraction Analyses—X-ray powder diffraction (XRD)patterns are shown in FIGS. 16-19. FIG. 16 shows the diffractionpatterns for the calcined zirconia, bulk WO₃ and tungsten oxide WO₃(40%)supported by zirconia prepared by impregnation and WO₃(40%) supported byzirconia prepared by co-precipitation. The phase identification,performed using MDI Jaded 6, was based on the chemical componentsinvolved and the Figure of Merits (FOM) of suggested phases. Firstly,the calcined commercial zirconia was analyzed to form a baseline for thesupported samples. The calcined zirconia diffraction pattern generallymatched the baddeleyite, syn-ZrO₂ phase (37-1484 JCPDS card) (JointCommittee on Powder Diffraction Standards). The bulk WO₃ diffractionpattern was in accordance with the characteristic pattern of themonoclinic structure of tungsten oxide (43-1035 JCPDS card). Thetungsten oxide and the baddeleyite (ZrO₂) phases appear in theWO₃(40%)/ZrO₂ sample prepared by impregnating commercial zirconia withammonium meta tungstate ((NH₄)₆(H₂W₁₂O₄₀)). The XRD pattern generatedfor the sample of (WO₃(40%)/ZrO₂) prepared by co-precipitation usingtungsten (VI) chloride (WCl₆) as a source of tungsten and zirconium (IV)oxynitrate hydrate (N₂O₇Zr.xH₂O) as a support precursor indicates thebaddeleyite phase along with the zirconium oxide phase (50-1089 JCPDScard) with no phases of WO₃ formed. Moreover, the peaks from theco-precipitation method are wider and weaker.

FIG. 17 shows a comparison of XRD patterns of theNiO(20%)-WO₃(20%)/ZrO₂-coprecipitation withNiO(20%)-WO₃(20%)ZrO₂-impregnation. Both the co-precipitated and theimpregnated samples do not indicate any formation of tungsten oxidephases. Bunsenite (NiO) and nickel tungsten oxide (NiWO₄) phases areobserved. The zirconia phase observed (50-1089 JCPDS) for theco-precipitated sample is different from that of the impregnated sample(baddeleyite, ZrO₂).

FIG. 18 shows the XRD patterns of the dual mixed metallic oxide catalystprepared by impregnation with loading of NiO kept constant at 20% andthe WO₃ loading varying from 5% to 35%, starting with the supportedNiO(20%)ZrO₂-impregnation. The NiO phase is present as a bunsenite phase(NiO) (47-1049 JCPDS card) and the zirconia phase is present as abaddeleyite phase. The NiO(20%)-WO₃(5%)//ZrO₂-impregnation sampledisplayed the baddeleyite phase (ZrO₂), the bunsenite phase (NiO) andthe tungsten oxide (WO₃) phase (41-0905 JCPDS card). Further WO₃ loading(NiO(20%)-WO₃(10%)/ZrO₂-impregnation) produced the same diffractionpeaks with the formation of nickel tungsten oxide (NiWO₄) as a new phase(15-0755 JCPDS card). NiO(20%)-WO₃(15%)/ZrO₂-impregnation showed samephases observed in the NiO(20%)-WO₃(10%)/ZrO₂-impregnation sample withless intense ZrO₂ and tungsten oxide diffraction peaks. TheNiO(20%)-WO₃(20%)/ZrO₂-impregnation sample has smaller peaks for thebaddeleyite phase (the ZrO₂ support) and no peaks for tungsten oxide.The bunsenite (NiO) peak and the nickel tungsten oxide (NiWO₄) peak bothhave relatively higher intensities. Phases similar to those observed inNiO(20%)-WO₃(20%)ZrO₂-impregnation were also observed inNiO(25%)-WO₃(20%)/ZrO₂-impregnation, NiO(30%)-WO₃(20%)ZrO₂-impregnation,and NiO(35%)-WO₃(20%)/ZrO₂-impregnation samples (see FIG. 19). However,the nickel tungsten oxide (NiWO₄) peaks were higher and stronger up to30% NiO loading, while the baddeleyite peaks became weaker withincreased loading of NiO. The NiO(20%)-WO₃(25%)/ZrO₂-impregnation sampleshowed the best performance with changing of the amount of WO₃ loading.This provided motivation to alter the amount of NiO loading whilekeeping the WO₃ loading constant at 25% (w/w), representing the optimalloading of WO₃.

The XRD patterns of the dual metal oxide catalysts prepared byimpregnation with constant WO₃ loading (25%) and varied NiO loading areshown in FIG. 19. For the NiO(5%)-WO₃(25%)ZrO₂-impregnation sample, thebaddeleyite, tungsten oxide and nickel tungsten oxide phases arevisible. The tungsten oxide phase disappears with increased NiO loading.It was observed that tungsten oxide is involved in the formation of thenickel tungsten oxide (NiWO₄) phase. ForNiO(20%)-WO₃(25%)/ZrO₂-impregnation, NiO(25%)-WO₃(25%)/ZrO₂-impregnationand NiO(60%)-WO₃(25%)/ZrO₂-impregnation samples, baddeleyite, bunsenite,and nickel tungsten oxide phases are visible. The baddeleyite phasepeaks become weaker with increased metal oxide loading, and these peaksbecome very weak and broad for the NiO(65%)-WO₃(25%)/ZrO₂-impregnationand NiO(70%)-WO₃(25%)/ZrO₂-impregnation samples. Furthermore, thebaddeleyite peaks disappear completely with NiO(75%)-WO₃(25%) because nosupport is present. On the other hand, the intensity of the bunsenitepeaks and the nickel tungsten oxide peaks increases with increasingamounts of NiO loading.

As previously reported by Tijani et. al,¹⁵ the thermally stable zirconiasupport (2715° C. melting point) does not show evidence of anyinteraction with the active metals, which facilitates the reduction ofthe metal oxides. However, upon increasing the loading of the WO₃ (to10% and more) in the dual metallic oxygen carrier with 20% nickelloading, the nickel and tungsten become involved in a strong interactionwith each other, resulting in formation of the NiWO₄ phase. The presenceof this phase is believed to be related to the enhancement of the oxygencarrying capacity of samples in which this phase exists. It has beensuggested that the NiWO₄ may be reduced by two different pathways⁷: (i)reduction of NiWO₄ into a NiWO_(x) compound, and (ii) decomposition ofNiWO₄ into NiO and WO₃ ⁺ (which similar to the bulk WO₃), which arefurther reduced to Ni and WO_(y). Among these two possibilities thedecomposition pathway appears to be the most probable.

Scherrer's equation (equation 4) was used to estimate the crystal sizeof the samples:

$\begin{matrix}{{\text{<}L\text{>}} = \frac{K\lambda}{\beta\cos\theta}} & \left( {{eq}\mspace{14mu} 4} \right)\end{matrix}$

Where: <L> is the measure of the particle dimension in the directionnormal to the reflecting planeK is a constant (usually taken as 1)λ is the wavelength of the X-rays used to excite the samples (0.15406nm)β is the width of the peak in radiansθ is the angle confined between the beam and the plane perpendicular tothe reflecting plane.

The FWHM (Full Width at Half Maximum) technique used to estimate thewidth of the peaks. The results are shown in Table 5.

TABLE 5 Crystallite Size Estimation Crystallite Sample Formed phasessize (nm) Calcined zirconia Baddeleyite (ZrO₂) 35^(a) Bulk WO₃ TungstenOxide 51^(b) (WO₃) WO₃(40%)/ZrO₂- Baddeleyite (ZrO₂) 58^(a) ImpregnationTungsten Oxide 65^(b) (WO₃) WO₃(40%)/ZrO₂- Baddeleyite (ZrO₂) 36^(a)Co-precipitation Zirconium Oxide 41^(c) (ZrO₂) NiO(20%)—WO₃(20%)/ZrO₂-Baddeleyite (ZrO₂) 43^(a) Impregnation Bunsenite (NiO) 27^(d) NickelTungsten 54^(f) Oxide (NiWO₄) NiO(20%)—WO₃(20%)/ZrO₂- Zirconium Oxide25^(c) Co-precipitation (ZrO₂) Bunsenite (NiO) 26^(d) Nickel Tungsten57^(f) Oxide (NiWO₄) NiO(20%)/ZrO₂- Baddeleyite (ZrO₂) 29^(a)Impregnation Bunsenite (NiO) 25^(d) NiO(20%)—WO₃(5%)/ZrO₂- Baddeleyite(ZrO₂) 38^(a) Impregnation Bunsenite (NiO) 26^(d) Tungsten Oxide 44^(e)(WO₃) NiO(20%)—WO₃(10%)/ZrO₂- Baddeleyite (ZrO₂) 42^(a) ImpregnationBunsenite (NiO) 28^(d) Tungsten Oxide 44^(e) (WO₃) Nickel Tungsten42^(f) Oxide (NiWO₄) NiO(20%)—WO₃(15%)/ZrO₂- Baddeleyite (ZrO₂) 34^(a)Impregnation Bunsenite (NiO) 26^(d) Tungsten Oxide 32^(e) (WO₃) NickelTungsten 33^(f) Oxide (NiWO₄) NiO(20%)—WO₃(20%)/ZrO₂- Baddeleyite (ZrO₂)43^(a) Impregnation Bunsenite (NiO) 27^(d) Nickel Tungsten 54^(f) Oxide(NiWO₄) NiO(20%)—WO₃(25%)/ZrO₂- Baddeleyite (ZrO₂) 47^(a) ImpregnationBunsenite (NiO) 27^(d) Nickel Tungsten 50^(f) Oxide (NiWO₄)NiO(20%)—WO₃(30%)/ZrO₂- Baddeleyite (ZrO₂) 38^(a) Impregnation Bunsenite(NiO) 24^(d) Nickel Tungsten 46^(f) Oxide (NiWO₄)NiO(20%)—WO₃(35%)/ZrO₂- Baddeleyite (ZrO₂) 38^(a) Impregnation Bunsenite(NiO) 21^(d) Nickel Tungsten 41^(f) Oxide (NiWO₄) NiO(5%)—WO₃(25%)/ZrO₂-Baddeleyite (ZrO₂) 45^(a) Impregnation Tungsten Oxide 48^(e) (WO₃)Nickel Tungsten 58^(f) Oxide (NiWO₄) NiO(25%)—WO₃(25%)/ZrO₂- Baddeleyite(ZrO₂) 38^(a) Impregnation Bunsenite (NiO) 25^(d) Nickel Tungsten 47^(f)Oxide (NiWO₄) NiO(60%)—WO₃(25%)/ZrO₂- Baddeleyite (ZrO₂) 44^(a)Impregnation Bunsenite (NiO) 27^(d) Nickel Tungsten 51^(f) Oxide (NiWO₄)NiO(65%)—WO₃(25%)/ZrO₂- Bunsenite (NiO) 29^(d) Impregnation NickelTungsten 47^(f) Oxide (NiWO₄) NiO(70%)—WO₃(25%)/ZrO₂- Bunsenite (NiO)30^(d) Impregnation Nickel Tungsten 65^(f) Oxide (NiWO₄)NiO(75%)—WO₃(25%)- Bunsenite (NiO) 32^(d) Impregnation Nickel Tungsten47^(f) Oxide (NiWO₄) ^(a)Crystallite/phase size estimated usingScherrer's equation on the (−1 1 1) plane of baddeleyite, syn-(ZrO₂37-1484) phase ^(b)Crystallite/phase size estimated using Scherrer'sequation on the (2 0 2) plane of tungsten oxide-(WO₃ 43-1035) phase^(c)Crystallite/phase size estimated using Scherrer's equation on the (01 1) plane of zirconium oxide-(ZrO₂ 50-1089) phase ^(d)Crystallite/phasesize estimated using Scherrer's equation on the (2 0 0) plane ofbunsenite, syn-(NiO 47-1049) phase ^(e)Crystallite/phase size estimatedusing Scherrer's equation on the (1 1 0) plane of tungsten oxide-(WO₃41-0905) phase ^(f)Crystallite/phase size estimated using Scherrer'sequation on the (−1 1 1) plane of nickel tungsten oxide-(NiWO₄ 15-0755)phase.

The sizes of the crystallites (nm) of the oxygen carrier catalysts arelisted in Table 5. The uncertainty involved in the estimation of theFWHM from MDI Jade 6 is less than 10% for all samples. For the tungstenoxide phase, the crystallite size is smaller in the supported samplesthan in the bulk metal oxide samples. Generally, the impregnated samplesformed by impregnation have higher crystallite/phase sizes than thesamples formed by co-precipitation, except for the NiWO₄ phase (54 nmimpregnated and 57 co-precipitated) which falls within the margin oferror. The crystal size for the tungsten oxide phase is about 44 nm forboth Ni(20%)-WO₃(5%)-ZrO₂-Impregnation andNi(20%)-WO₃(10%)-ZrO₂-Impregnation samples. With further loading of WO₃(i.e. Ni(20%)-WO₃(15%)-ZrO₂-Impregnation), the crystallite size isreduced to 32 nm before the disappearance of the phase withNi(20%)-WO₃(20%)-ZrO₂-Impregnation sample and samples loaded with moreWO₃. For the oxygen carriers with constant WO₃ loading of 25%, anddifferent nickel oxide loading, the crystallite size of the nickeltungsten oxide phase was between 47-58 nm, while the size for thebunsenite phase was between 25-32 nm. It was also observed that as theNiO loading increases, the crystallite size of this phase alsoincreases.

EQUIVALENTS AND SCOPE

Other than described herein, or unless otherwise expressly specified,all of the numerical ranges, amounts, values and percentages, such asthose for amounts of materials, elemental contents, times andtemperatures, ratios of amounts, and others, in the following portion ofthe specification and attached claims may be read as if prefaced by theword “about” even though the term “about” may not expressly appear withthe value, amount, or range. Accordingly, unless indicated to thecontrary, the numerical parameters set forth in the followingspecification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained. At the veryleast, and not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

Any patent, publication, internet site, or other disclosure material, inwhole or in part, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as explicitly set forth hereinsupersedes any conflicting material incorporated herein by reference.Any material, or portion thereof, that is said to be incorporated byreference herein, but which conflicts with existing definitions,statements, or other disclosure material set forth herein will only beincorporated to the extent that no conflict arises between thatincorporated material and the existing disclosure material.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art.

It will be understood by those skilled in the art that various changesin form and details may be made to the embodiments described thereinwithout departing from the scope of the invention encompassed by theappended claims.

In the claims, articles such as “a,” “an,” and “the” may mean one ormore than one unless indicated to the contrary or otherwise evident fromthe context. Claims or descriptions that include “or” between one ormore members of a group are considered satisfied if one, more than one,or all of the group members are present in, employed in, or otherwiserelevant to a given product or process unless indicated to the contraryor otherwise evident from the context.

It is also noted that the term “comprising” is intended to be open andpermits but does not require the inclusion of additional elements orsteps. When the term “comprising” is used herein, the term “consistingof” is thus also encompassed and disclosed. Where ranges are given,endpoints are included. Furthermore, it is to be understood that unlessotherwise indicated or otherwise evident from the context andunderstanding of one of ordinary skill in the art, values that areexpressed as ranges can assume any specific value or subrange within thestated ranges in different embodiments of the invention, to the tenth ofthe unit of the lower limit of the range, unless the context clearlydictates otherwise. Where the term “about” is used, it is understood toreflect +/−10% of the recited value. In addition, it is to be understoodthat any particular embodiment of the present invention that fallswithin the prior art may be explicitly excluded from any one or more ofthe claims. Since such embodiments are deemed to be known to one ofordinary skill in the art, they may be excluded even if the exclusion isnot set forth explicitly herein.

REFERENCES

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1. A catalyst for use in chemical looping combustion, the catalystcomprising a mixture of metal oxides dispersed on a ceramic support, themixture of metal oxides forming a nickel tungsten oxide (NiWO₄)interaction complex which functions as an oxygen carrier in the chemicallooping combustion reaction.
 2. The catalyst of claim 1, wherein theceramic support is calcium aluminate of formula CaAl₂O₄, silica offormula SiO₂, titanium dioxide of formula TiO₂, perovskite of formulaCaTiO₃, alumina of formula Al₂O₃, yttrium dioxide of formula Y₂O₃,barium zirconate of formula BaZrO₃, magnesium aluminate of formulaMgAl₂O₄, magnesium silicate of formula MgSi₂O₄, lanthanum oxide offormula La₂O₃ or zirconia of formula ZrO₂.
 3. The catalyst of claim 1,wherein the ceramic support is zirconia of formula ZrO₂.
 4. The catalystof claim 3, wherein the zirconia is calcined at a temperature at orabove about 900° C. for at least about 4 hours.
 5. The catalyst of claim1, wherein the mixture of metal oxides includes nickel oxide of formulaNiO and tungsten oxide of formula WO₃.
 6. The catalyst of claim 5,comprising between about 25% to about 60% NiO (w/w), between about 10%to about 35% WO₃ (w/w) and between about 5% to about 65% ZrO₂ (w/w). 7.The catalyst of claim 1, having an oxygen carrying capacity of about4.2% (w/w) to about 15.6% (w/w).
 8. The catalyst of claim 1, having aBrunauer-Emmett-Teller (BET) surface area between about 4.1 m²/g toabout 16.7 m²/g.
 9. The catalyst of claim 1, having a pore volume ofabout 0.030 cm³/g to about 0.094 cm³/g.
 10. The catalyst of claim 1,having an adsorption average pore width (4V/A by BET) between about 225Å to about 300 Å.
 11. A process for synthesizing an oxygen carriercatalyst, the process comprising: mixing nickel (II) nitrate hexahydrate(N₂NiO₆.6H₂O) with ammonium metatungstate and a ceramic support in waterand evaporating the water.
 12. The process of claim 11, wherein theceramic support is calcium aluminate of formula CaAl₂O₄, silica offormula SiO₂, titanium dioxide of formula TiO₂, perovskite of formulaCaTiO₃, alumina of formula Al₂O₃, yttrium dioxide of formula Y₂O₃,barium zirconate of formula BaZrO₃, magnesium aluminate of formulaMgAl₂O₄, magnesium silicate of formula MgSi₂O₄, lanthanum oxide offormula La₂O₃ or zirconia of formula ZrO₂.
 13. The process of claim 11,wherein the ceramic support is zirconia of formula ZrO₂.
 14. The processof claim 13, wherein the zirconia is calcined at a temperature at orabove about 900° C. for at least about 4 hours.
 15. The process of claim11, wherein the synthesized catalyst includes nickel oxide of formulaNiO and tungsten oxide of formula WO₃.
 16. The process of claim 15,wherein the synthesized catalyst includes between about 25% to about 60%NiO (w/w), between about 10% to about 35% WO₃ (w/w) and between about 5%to about 65% ZrO₂ (w/w).
 17. The process of claim 11, wherein thesynthesized catalyst has an oxygen carrying capacity of about 4.2% (w/w)to about 15.6% (w/w).
 18. The process of claim 11, wherein thesynthesized catalyst has a Brunauer-Emmett-Teller (BET) surface areabetween about 4.1 m²/g to about 16.7 m²/g.
 19. The process of claim 11,wherein the synthesized catalyst has a pore volume of about 0.030 cm³/gto about 0.094 cm³/g.
 20. The process of claim 11, wherein thesynthesized catalyst has an adsorption average pore width (4V/A by BET)between about 225 Å to about 300 Å.